During the early 1900's, the technology was developed by Soderberg for the production of a continuous carbon electrode. The Soderberg process produces a self-baking electrode that is formed continuously from a soft carbon mixture that is baked in the same furnace in which it is used.
The Soderberg process is continuous, involving the addition of a paste to the upper end of the electrode-manufacturing apparatus. This paste is a mixture of a carbonaceous aggregate and a pitch binder. The paste has sufficient mobility to travel through the temperature gradient. The temperature increases as the paste approaches the electrolyte cell area. During this travel, the paste typically begins to harden into something akin to a jelly formation with loss of volatiles--the paste retaining thermoplasticity while not being too fluid to cause excessive overflow of the paste. The moving paste then approaches the use zone where it hardens and attains electrode integrity through densification and binder graphitization upon exposure to the operating temperatures of the electrolytic cell. In the cell, the electrode is not only used but is continually consumed.
The apparatus for making a Soderberg electrode consists of a ribbed cylinder of thin sheet iron into which the electrode paste is filled. Thus, the upper end of the equipment is filled with raw paste. The formed electrode that projects from the lower end of the Soderberg equipment is continuously consumed in the furnace. From time to time, the equipment is operated to permit the electrode, that has been formed in the equipment, to drop downwardly to replace the amount of electrode that has been consumed. This permits the paste in the upper end of the equipment to drop downwardly. During this process, the paste drops through a zone of increasing temperature, and the heat from the furnace bakes the paste. The material in the Soderberg equipment thus consists of an upper portion that is in mobile paste form, a lower portion that is a baked, hard electrode, and an intermediate portion in which the paste is gradually changing from mobile paste form to hard baked electrode form. For convenience, this three part structure will be referred to in this application as the electrode, although strictly speaking, only the baked lower portion functions as an electrode.
The cylindrical holder part of the Soderberg apparatus is generally of the water-cooled clasp type. It is designed to permit the electrode to move gradually at a predetermined rate or to slip down from time to time, and thus, to bring unbaked portions of the paste to higher and higher temperatures as they get closer and closer to the hot electrolytic cell zone. Slipping is effected by loosening the grip of the holder until the electrode slides down by its own weight. This manipulation of the electrode may be effected from a place situated at some distance from the furnace. Slipping of the electrode is ordinarily carried out under full load. Operation of the electrode is maintained during the entire process, so that current continues to pass through the electrode. The electrical contact is maintained by means of a sliding contact between the holder and the electrode casing.
In this art, the term "anode" is used where the carbon or graphite baked electrode is used as an anode in an actual electrolysis, as in the manufacture of aluminum or in electrolysis, as in the electrolysis of brine; and the term "electrode" refers to a generally similar article used for an application where the function is primarily carrying electrical current.
The rate at which a Soderberg electrode is consumed depends on the particular application for which it is being used. For example, in furnaces for the production of calcium carbide and ferro alloys, the feed rate may be in the range from about 4 inches to about 20 inches per day. Generally, the materials used in the manufacture of a Soderberg electrode are calcined anthracite, alone or with calcined petroleum coke, and a medium pitch.
Soderberg electrodes are used in several different applications. In the electrolytic production of aluminum, the anodes are predominantly produced by the prebaked process. However, substantial use of the Soderberg process still prevails in aluminum manufacturing operations around the world. The anode paste is ordinarily made from a base of petroleum coke.
The Soderberg process may also be used for making electrodes for use in electric furnaces. Generally these are what can be considered to be artificial graphite electrodes. The paste for such electrodes is made from low ash coal or petroleum coke. These materials are crushed and calcined to remove volatile matter, then mixed with tar or pitch as a binder. The resulting paste can then be used in the Soderberg process to make the desired electrode, or alternatively, the paste may be molded or extruded or pressed to produce a green electrode that is baked, then cleaned, and either used as such or machined to a desired shape for a specific use. Electrodes so produced, in general, are inferior in performance compared to the prebaked electrodes in their densities, plasticity, electrical properties and the like.
Soderberg electrodes are used principally in aluminum reduction, calcium carbide production, the production of electric pig iron, processing copper matte, and other kinds of ferro alloy and smelting operations. They are also used in the manufacture of phosphorus.
The green raw filler materials from which Soderberg electrodes are made may have a volatiles content in the range from 5% to 15%. Thus, for petroleum coke, 10% is typical. In the calcination operation, hydrocarbons are removed and the coke shrinks in volume, resulting in a density increase. The weight loss may be 25% or higher. If the raw material is not calcined, the release of volatiles and the shrinkage take place during electrode formation and may result in cracked structures.
In one Soderberg process that is currently in use, the electrode paste is made from electrically calcined anthracite, with coal tar based pitch and anthracene oil as the binder. In the formulation of the paste, the calcined anthracite normally amounts to from about 65% to about 70% by weight of the paste, the balance being provided by the coal tar pitch and the anthracene oil.
Coal tar pitch levels as high as 30% to 35% by weight of the paste are commonly used in binders in the Soderberg process. Such a high pitch content leads to extensive fume evolution and to a loss of volatiles, which in turn lead to electrodes having poor integrity. Additionally, coal tar pitch has been under heavy environmental pressures and scrutiny for carcinogenicity attributed to volatile components such as "pyrene". The evolution of these components are more predominantly visible (i.e. apparent) in the Soderberg process mainly due to the heavy exposure of the work force to such fumes in the Soderberg process.
Thus, operations using the Soderberg process are under heavy pressures to either install extensive and cost prohibitive environmental control measures or to resort to alternative processes such as the "Prebaked Electrode Process" which requires additional capital investment, extensive equipment renovation and retrofitting, or resort to more environmentally safe binder systems while not sacrificing the carbon efficiencies of the binder electrode. These drawbacks have turned the Soderberg process away from favor. However, even with pre-baked anodes, there are drawbacks such as porosity which is a major problem that requires strict controls to avoid oxidation and maximize carbon utilization.
To avoid these drawbacks, some furfuryl alcohol and/or furfural resins have been suggested for use in pitch binders in making Soderberg electrodes. Generally a resin would not only function as pitch does in a paste, that is, as a sacrificial binder, but a properly selected resin (binder) should while retaining the plasticity properties of coal tar pitch also contribute similar residual carbon values and structure upon hardening and pyrolysis. Furfuryl alcohol and furfural resins result in thermoset structures in the 200.degree. F. (93.degree. C.) to 500.degree. F. (260.degree. C.) range and lack in any flow and/or plasticity beyond this temperature. Additionally, both furfural and furfuryl alcohol are volatile monomers and the fumes are both noxious and toxic. Consequently, successful use of said binders in commercial applications is very limited.
The paste that is used in the conventional manufacture of Soderberg electrodes can be poured directly into the electrode equipment. Alternatively, it can be cast into blocks or other shapes for shipment and use. At room temperature, such blocks are hard and are easy to handle. When such blocks are heated to about 250.degree. F., the pasty consistency is restored so that the paste can flow readily as required by the Soderberg equipment.
In summary, the Soderberg process of anode manufacture is an old method and is now generally considered economically inefficient. However, aluminum manufacturers around the world still have several operations producing aluminum using the Soderberg process. The Soderberg process anodes are using pitch at about 25-30% binder level and are plagued by environmental problems. The problems are mainly in the area of toxic vapors produced at the electrolyte cell area. The vapor control and adequate protection are issues that must be dealt with. The industry is faced with two alternatives: installing extensive pollution control involving substantial capital investments or exploring alternative environmentally safe and efficient binders.